1. Field of the Invention
The present invention relates to a timing recovery circuit and a timing recovery method, and more particularly, to a timing recovery circuit and a timing recovery method of taking a modulo input signal as a datum point for implementing timing recovery.
2. Description of the Prior Art
In certain communication systems, a technique named Tomlinson-Harashima precoding (THP) is utilized at the transmitting terminals of the communication systems for implementing timing recovery. For example, THP may be utilized in communication systems based on 10G Base-T Ethernet applied with IEEE 802.3an. THP includes placing a feedback filter of a decision feedback equalizer at a transmitting terminal of a communication system instead of at a receiving terminal of the communication system for reducing error propagation resulted from symbol errors of a slicer in advance. However, for restricting symbol values of signals at the transmitting terminal so as to reduce symbol errors, a modulo processing circuit is further disposed at the transmitting terminal for implementing such restrictions. Note that a weighted modulo of the modulo processing circuit is 2M, where M is a modulo. Therefore, an output signal Tx_output at the transmitting terminal may be indicated as follows:Tx_output=(FIR_output+M)mod(2M)−M  (1)where FIR_output indicates a symbol value of finite impulse response (FIR) in the feedback equalizer. Considering the abovementioned 10G Base-T Ethernet, when the feedback equalizer is assumed to take FIR of 16 taps, a value of the corresponding modulo M is 16, and the output signal Tx_output at the transmitting terminal may be indicated as follows:Tx_output=(FIR_output+16)mod(32)−16  (2)
Since the modulo processing circuit having the modulo value 2M has been applied at the transmitting terminal of the communication system, another modulo processing circuit having the modulo value 2M is also required to be applied before a slicer of the receiving terminal of the communication system for recovering transmitted signals of the transmitting terminal. However, an obvious fault may easily happen in the communication system utilizing THP. Since the communication system utilizing THP represents signals with a pulse amplitude modulation (PAM) based on a value 16, i.e., PAM 16, symbol values of the represented signals include ±1, ±3, ±5, ±7, ±9, ±13, ±15. When a transmitted signal having a symbol value of +15 is interfered by noises in the channel so that a corresponding received signals has a symbol value of 16.5, a recovered signal having a symbol value of −15 is retrieved from the slicer after obtaining a symbol value of 15.5 according to the equation (2). In other words, since the noises in the channel merely results in a shift symbol value of +1.5 on the transmitted signal, a shift symbol value in the recovered signal is significantly raised to −30 because THP is utilized in both the transmitting terminal and the receiving terminal of the communication system. A probability that the symbol value +15 happens in the signal is ⅛, therefore, a huge amount of symbol errors appears accordingly while mass communication and related timing recovery is performed. In conclusion, symbol errors of ±2·(M−1) are easily resulted while THP, which takes input signals of a slicer as datum points, is utilized for implementing timing recovery. And as a consequence, the implemented timing recovery cannot precisely synchronize signals at the receiving terminal with the signals at the transmitting terminal. And even in certain related prior arts, signals having the symbol value ±2·(M−1) are directly eliminated in the modulo processing circuit having a modulo value 2M, however, the eliminated signals results in aliasing of larger degrees as well.